Bag bone - Taikotsu (English spelling) os marsupiale

Japanese: 袋骨 - たいこつ（英語表記）os marsupiale

The proximal ends of the hind limbs of mammals articulate with a box-shaped structure (pelvis) made up of the bilaterally symmetrical ilium, pubis, and ischium, and part of the vertebral column (sacrum). Monotremes and marsupials have a pair of plate-like bones that extend forward and outward, like goat horns, loosely attached to the anterior (superior) edge of the pubis that make up the pelvis, and are embedded in the abdominal muscles. In other words, this flat bone serves to reinforce the abdominal muscles of the animal, and is important for female marsupials that have a brood pouch in their abdomen. Because of this fact, this bone was named the bladder bone, but since it is also found in males that do not have a brood pouch and in marsupials that do not have one, it is more correctly called the prepubis (or suprapubis).

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

哺乳類の後肢の近位端は，左右相称な腸骨，恥骨および坐骨と脊柱の一部(仙骨)とからできた箱形の構造物(骨盤)と関節する。単孔類と有袋類には，この骨盤を構成する恥骨の前縁(上縁)にゆるく結合するヤギの角のような形の前外方へ延びる1対の板状の骨があり，腹筋の中へ埋まっている。つまり，この扁平な骨は動物の腹筋を補強する役割を果たし，腹部に育児囊がある有袋類の雌にとっては重要なものである。この事実から，この骨は袋骨と名づけられたが，育児囊をもたぬ雄やそれのない有袋類にも認められるので，前恥骨(または上恥骨)と呼ぶのが正しい。

出典　株式会社平凡社世界大百科事典第２版について　情報

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